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SubjectRe: [PATCH 4/9] x86/alternative: Implement .retpoline_sites support
On Wed, Oct 13, 2021 at 02:49:10PM -0700, Josh Poimboeuf wrote:
> On Wed, Oct 13, 2021 at 11:20:02PM +0200, Peter Zijlstra wrote:
> > On Wed, Oct 13, 2021 at 01:39:27PM -0700, Josh Poimboeuf wrote:
> > > On Wed, Oct 13, 2021 at 02:22:21PM +0200, Peter Zijlstra wrote:
> > > > +static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
> > > > +{
> > > > + void (*target)(void);
> > > > + int reg, i = 0;
> > > > +
> > > > + if (cpu_feature_enabled(X86_FEATURE_RETPOLINE))
> > > > + return -1;
> > >
> > > Better to do this check further up the call stack in apply_retpolines()
> > > before looping through all the call sites?
> >
> > In fact, I've pushed it further down, in order to always validate the
> > absense of rsp.
> >
> > > > +
> > > > + target = addr + insn->length + insn->immediate.value;
> > > > + reg = (target - &__x86_indirect_thunk_rax) /
> > > > + (&__x86_indirect_thunk_rcx - &__x86_indirect_thunk_rax);
> > > > +
> > > > + if (WARN_ON_ONCE(reg & ~0xf))
> > > > + return -1;
> > >
> > > It would be more robust and less magical to just have a basic lookup
> > > table array which converts a thunk address to a reg. Then you can just
> > > avoid all the safety checks because it's no longer insane ;-)
> >
> > Andrew suggested the reverse lookup to validate. That should give the
> > same robustness but lacks the linear lookup.
>
> So you've got a WARN_ON_ONCE, a BUG_ON, and a too-deep feature check,
> all in the name of supporting this scheme. ok...

Not to mention the extra silly rsp thunks...

> If performance of the linear lookup were a real concern then you could
> just put rax and r11 at the beginning of the array.

--
Josh

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